High power electric discharge gas lasers can be operated to provide high pulsed output energy. To produce such energy, the gas within the laser is usually excited by means of an intense source of electron excitation, either from a high voltage self sustained electric discharge, or, an electron beam sustained discharge. The excitation of the gas or gases within the laser results in the emission of light energy of the appropriate wave length that can then be collimated as is well known in the art. In practice, the self-sustained electric discharge method is the popular alternative.
To provide a self-sustained discharge in a gas laser, one must first preionize the gas by means of generating a low level of electrons in the discharge gap, then avalanching the low-level ionization to the final level required for breakdown of the gas, and finally excite the gas using sufficient energy to sustain the discharge.
In the past, single, high voltage pulses having fast rise times and a discharge pulse duration in the order of tens to hundreds of nanoseconds or more have been used to avalanche ionize the gas and then to provide the required sustained discharge. Low inductance rail-gap, thyratron and magnetic switches together with pulse forming networks (PFN) have been used to provide such high energy, rapid rise times and long duration pulses. However, in terms of switches, magnetic switching using saturable inductors are preferable as these switches offer long life, reliability, low cost, less complexity, and high repetition rate capability.
U.S. Pat. No. 4,698,518 entitled Magnetically Switched Power Supply System for Lasers in the name of Pacala teaches the use of saturable inductors as switches for compressing the width and sharpening the rise time of pulses from high voltage, high impedance pulse generators to provide the necessary excitation to EDGLs. Taylor (the applicant) et al. in U.S. Pat. No. 4,679,203 teach saturation of a pulse transformer core inductance to efficiently switch a main energy store into the discharge after the breakdown of the laser gas mix has occurred.
The use of magnetic-spiker sustainer excitation circuits in rare gas halide lasers such as XeCl lasers has resulted in more efficient lasers having longer optical pulse duration, and a higher beam quality compared to lasers with conventional electrical discharge excitation circuits. Magnetic-spiker sustainer circuits are currently being investigated for the generation of very high average power (1 kW) XeCl lasers for use in materials processing applications. As well, these circuits are capable of producing microsecond duration optical pulses. Long optical pulse duration provides for an increased number of laser cavity round trips, thereby providing conditions that allow for more direct control on the laser divergence, line width, polarization and level of amplified spontaneous emission. The low peak power associated with long optical pulse operation is advantageous for applications where non linear effects such as material damage must be avoided. One such example is in the medical field of XeCl laser coronary angioplasty, where long optical pulses are used to avoid surface damage to the fiber optic delivery system.
Spiker-sustainer circuits are comprised of two electrical circuits; a spiker circuit generates a fast high voltage pulse to initiate gas breakdown; a main energy storage circuit that is generally charged to a low voltage can be adjusted to ensure good energy transfer into the discharge. In magnetic-spiker circuits, the saturable magnetic material or core provides electrical isolation between the spiker circuit and the main energy storage circuit.
There are three basic modes of operation of magnetic-spiker circuits depending on the magnitude and polarity of the spiker voltage relative to the voltage on the main energy storage. When the polarity of the switched spiker voltage and the main energy storage are the same, the circuits operate in the diode mode. When the polarity of the switched spiker voltage and the main energy storage are opposite, the circuit operates in the switch mode. The overshoot mode is an adaptation of the switch mode. Although fine tuning of circuit parameters is required to obtain optimum laser performance in any of the modes, in some respects, operating in the overshoot has been found to be advantageous. Furthermore, operating in a modified overshoot mode (MOM) which will be described hereafter provides even greater advantages.